Although homopolymers and copolymers of 1,4-dioxepan2-one have been described in the literature, very little is known about the physical properties of these polymers. The first mention of homopolymers of 1,4-dioxepan-2-one was by Spanagel in U.S. Pat. No. 2,163,109 issued on Jun. 20, 1939, which cites a German article authored by Palomaa et al. in Ber. 66, 1629, (1933). Spanagel described the synthesis of these homopolymers from hydroxyether acids using a metal catalyst. These homopolymers were depolymerized after synthesis into lactones for use in perfumes. The physical properties of this polymer were not described.
More recently 1,4-dioxepan-2-one homopolymers and copolymers were described by Doddi et al. in U.S. Pat. No. 4,052,988. Doddi described the synthesis of homopolymers and copolymers of 1,4-dioxepan-2-one for use as absorbable synthetic sutures, tendons and the like. The copolymers disclosed by Doddi et al. were described as containing predominately 1,4-dioxepan-2-one and up to 50 weight percent of another copolymerizable monomer such as lactide or glycolide.
Similarly, European Patent Application, EPA 460,428 A2, also describes copolymers of 1,4-dioxepan-2-one and other fast reacting monomers such as glycolide and lactide. This application describes a block copolymer formed by a two stage polymerization process. In the first stage of this process, a prepolymer is formed containing predominately a monomer such as 1,4-dioxepan-2-one, the remainder of the prepolymer being a monomer such as glycolide or lactide. In the second stage of the polymerization, the prepolymer is reacted with an additional lactone monomer to provide a segmented block copolymer. Unfortunately, neither Doddi or EPA 460 428 A2 describes the physical properties of polymers containing 1,4-dioxepan-2-one.
The structural isomer of 1,4-dioxepan-2-one, namely 1,5-dioxepan-2-one, has also been studied. U.S. Pat. Nos. 4,190,720 and 4,470,416 describe copolymers of 1,5-dioxepan-2-one and .epsilon.-caprolactone, glycolide, or lactide. In addition, the homopolymerization of 1,5-dioxepan-2-one and its cyclic dimer has been investigated. Albersson et al. (Macromolecules 22, 3838-3846, 1989) have polymerized 1,5-dioxepan-2-one and its cyclic dimer. The resulting poly[1,5-dioxepan-2-one]was completely amorphous with a glass transition temperature of -39.degree. C. Since poly[1,5-dioxepan-2-one]is an amorphous elastomer, it can only be used as an absorbable toughening agent either as a discreet phase in a polymer blend or composite, or as a segment in a block copolymer.
Surprisingly, we have discovered that poly[1,4-dioxepan-2-one]is a semicrystalline polymer. In addition, contrary to the teachings of Doddi, poly[1,4-dioxepan-2one]crystallizes too slowly to be melt spun into fibers. Due to the slow crystallization rates of poly[1,4-dioxepan-2-one], it is desirable to limit the amount of 1,4-dioxepan-2-one that is incorporated into copolymers to less than 45 weight percent. However, poly[1,4-dioxepan-2-one]unlike the amorphous poly[1,5-dioxepan-2-one]be used in biomedical applications that require greater dimensional stability (such as molded parts or sutures). When poly[1,4-dioxepan-2-one]is employed in a polymer blend or composite, it would exist as a discreet crystalline phase; in a block copolymer, it would also exist as a discreet crystallizable segment that would tend to phase separate and crystallize into distinct phases. These copolymers, blends, or composites would be composed of at least two crystalline regions; one region being rich in poly[1,4-dioxepan-2-one]. Consequently, polymeric materials which incorporate the crystallizable poly[1,4-dioxepan-2-one]would exhibit new biophysical properties such as breaking strength retention and absorption profiles not achieved with its structural analog.